1. Field of the Invention
The present invention relates to a scanning optical system and an image forming apparatus using the same and, more particularly, to a scanning optical system suited to an apparatus, such as a laser beam printer (LBP) or digital copying machine, which makes the light beam emitted from a light source means strike a light deflector at a predetermined angle with respect to a plane perpendicular to the rotation axis of the light deflector, deflects the light beam, and scans it on a scanned surface, thereby recording image information.
2. Related Background Art
Various scanning optical systems are disclosed in, for example, Japanese Patent Application Laid-open Nos. 7-27991 and 9-230274, which make the light beam emitted from a light source means strike a polygon mirror as a light deflector at a predetermined angle with respect to a plane perpendicular to the rotation axis of the light deflector, deflect/reflect the light beam, and scan it on a scanned surface, thereby recording image information.
According to Japanese Patent Application Laid-open No. 7-27991, in the scanning optical system which makes the light beam emitted from the light source means strike the polygon mirror at a predetermined angle with respect to a plane perpendicular to the rotation axis of the polygon mirror, deflects/reflects the light beam, and scans it on a scanned surface through a scanning lens, thereby recording image information, a post objective optical system is disclosed, in which a main scanning cross-section of a deflection surface (polygon surface) of the polygon mirror has an elliptic shape, and only the incident surface of the scanning lens is shaped to be displaced in the sub-scanning direction (a direction perpendicular to a main scanning plane which is a plane scanned by the light beam deflected/reflected by the polygon mirror).
According to Japanese Patent Application Laid-open No. 9-230274, in the scanning optical system which makes the light beam emitted from the light source means strike the polygon mirror at a predetermined angle with respect to a plane perpendicular to the rotation axis of the polygon mirror, deflects/reflects the light beam, and scans it on a scanned surface through a cylindrical lens or cylindrical mirror, thereby recording image information, an optical system is disclosed, in which the cylindrical lens or cylindrical mirror is positioned to make the light beam to be incident on the cylindrical lens or cylindrical mirror it at a predetermined height away from the baseline.
However, various problems are posed in the above conventional scanning optical systems, as described below.
The scanning optical system disclosed in Japanese Patent Application Laid-open No. 7-27991 is the post objective optical system in which the light beam emitted from the light source strikes the polygon mirror at a predetermined angle with respect to a plane perpendicular to the rotation axis of the polygon mirror, and the light beam is incident on the polygon mirror from the center in the deflection angle of the polygon mirror in the main scanning direction. A main scanning cross-section of a deflection surface of the polygon mirror is formed into an elliptic shape, and the incident surface of the scanning lens is shaped to be displaced in the sub-scanning direction.
In an optical system using such an incidence method, i.e., a so-called sub-scanning oblique incident optical system, the surface formed by the light beam deflected/reflected by the deflection surfaces does not become a plane but becomes a conical surface. When such a light beam strikes the scanning lens, the light beam incident positions shift from each other on the central portion and two end portions of the lens in the sub-scanning direction. In this reference, the resultant spot shape deformation is corrected by displacing the incident surface of the scanning lens in the sub-scanning direction, but no correction is made for fxcex8 characteristics. Therefore, the fxcex8 characteristics are corrected by continuously changing the oscillation timing of the semiconductor laser. If, however, such a correction is performed, since the scanning speed on the scanned surface is not constant, the amount of light on the scanned surface varies. If the emission time is continuously changed as well as the oscillation timing to keep the amount of light uniform, the apparent spot diameter changes in the main scanning direction. This makes it difficult to obtain good optical performance. Furthermore, since an elliptic polygon mirror is used as a deflection surface, the cost required to process mirror surfaces is high.
According to Japanese Patent Application Laid-open No. 9-230274, curvature of field is corrected by positioning the cylindrical lens or cylindrical mirror to make a light beam strike the cylindrical lens or cylindrical mirror at a predetermined height away from the baseline. However, no consideration is given to the above spot shape deformation. In addition, in a scanning optical system having an arrangement like the one disclosed in the reference, satisfactory optical performance cannot be obtained only by simply correcting curvature of field alone, because spot shape deformation like the one described above occurs. In addition, in an over field type scanning optical system like the one disclosed in this reference, in order to minimize the above spot shape deformation, set a small oblique incident angle in the sub-scanning direction so as to minimize light amount variations, spot diameter changes, and the like throughout the scanning range, and send a light beam onto the polygon mirror from substantially the center in the deflection angle of the polygon mirror, an arrangement a (so-called double path) is often employed, in which the light beam to be incident on the polygon mirror is temporarily made to strike the scanning lens, and the light beam deflected/reflected by a deflection surface of the polygon mirror is incident on the scanning lens again. In this arrangement, in which the light beam passes through the scanning lens twice, an image plane greatly moves in the main scanning direction unless the surface precision of the scanning lens is strictly managed. In an over field type double path optical system like the one disclosed in this reference, therefore, a scanning lens is made up of about two optical glass members which can be formed with high precision relatively easily and are resistant to environmental variations. However, a plastic material which is inexpensive but inferior to a glass material in environmental characteristics cannot be used for a scanning lens.
It is an object of the present invention to provide an optical scanning system in which a reduction in cost and proper correction of fxcex8 characteristics, curvature of field, and the like are realized by forming a scanning lens as a second optical system using one plastic lens in a sub-scanning oblique incident optical system, and more specifically, an over field scanning optical system, and properly setting the shape of the lens, spot deformation due to oblique incidence of light in the sub-scanning direction is effectively corrected by forming a correction lens as a third optical system by using a single plastic lens and optimizing its shape, position, and the like, and a diffraction optical element is formed in at least the first or second optical system, thereby realizing high environmental resistance and high quality, and an image forming apparatus using the same.
A scanning optical system of the present invention comprises:
a first optical system for shaping a light beam emitted from light source means and forming the light beam into a linear image elongated in the same direction as a main scanning direction;
a light deflector which has a deflection surface near an imaging position of the first optical system and deflects/scans the incident light beam in the main scanning direction;
a second optical system for forming the light beam deflected by the light deflector into an image on a scanned surface in the main scanning direction; and
a third optical system for forming the light beam deflected by the light deflector into an image on the scanned surface in a sub-scanning direction and setting the deflection surface of the light deflector and the scanned surface optically conjugate with each other, and is characterized in that the light beam guided by the first optical system is made to strike the light deflector at a predetermined angle with respect to a plane perpendicular to a rotation axis of the light deflector, and
a shape of at least one surface of a main scanning cross-section of a lens constituting the second optical system is formed into a nonarcuated shape, and a diffraction optical element having a refracting power at least in the main scanning direction is formed in at least one of the first and second optical systems.
In addition, the scanning optical system of the present invention is characterized in that
the light beam incident on the light deflector is incident from a substantially center in a deflection angle of the light deflector with a width of the light beam exceeding a width of the deflection surface of the light deflector in the main scanning direction,
each of the second and third optical systems comprises one lens,
a material for the single lens constituting the second optical system comprises a plastic material,
a material for the single lens constituting the third optical system comprises a plastic material,
the diffraction optical element is formed in the first optical system,
the diffraction optical element is formed in the second optical system,
|xcfx862S|xe2x89xa60.001 where xcfx862S is a refracting power of the single lens constituting the second optical system in the sub-scanning direction,
|xcfx863M|xe2x89xa60.001 where xcfx863M is a refracting power of the single lens constituting the third optical system in the main scanning direction,
the diffraction optical element functions to cancel out an aberration variation, caused in the scanning optical system by an environmental variation, by using a variation in wavelength of the light source means due to the environmental variation,
the single lens constituting the third optical system is located closer to the scanned surface than a middle position in a distance from the light deflector to the scanned surface,
a radius of curvature of at least one surface of a sub-scanning cross-section of the single lens constituting the third optical system continuously changes with distance from a lens optical axis in the main scanning direction,
the single lens constituting the third optical system is positioned to make a light beam strike the lens at a position shifted from a surface vertex in a sub-scanning cross-section of the lens by a predetermined amount in the sub-scanning direction,
the single lens constituting the third optical system is positioned such that in a sub-scanning cross-section of the lens a light beam incident on the lens makes a predetermined angle with an optical axis of the lens in the sub-scanning cross-section,
the single lens constituting the second optical system also has a function of the first optical system,
two surfaces of a sub-scanning cross-section of the single lens constituting the second optical system are formed into flat shapes,
two surfaces of a main scanning cross-section of the single lens constituting the third optical system are formed into arcuated shapes, or
the system further comprises a bending mirror inserted in an optical path between the light source means and the light deflector.
An image forming apparatus of the present invention is characterized by forming an image by using the scanning optical system described above.